1. Field of the Invention
The present invention relates to a cylindrical motor suitable for being formed in a small size.
2. Related Background Art
FIG. 7A is a schematic longitudinal sectional view showing one construction example of a conventional step motor, and FIG. 7B is a partial sectional view schematically showing a state of magnetic flux flowing from a stator of the step motor shown in FIG. 7A.
In FIG. 7A, two bobbins 101, each having a stator coil 105 wound concentrically on the bobbin 101, are arranged in the axial direction, and each of the two bobbins 101 is fixedly held by a separate stator yoke 106. On the inside diameter surface of the stator yoke 106, stator teeth 106a and 106b, which are arranged alternately in the circumferential direction on the inside diameter surface of the bobbin 101, are formed. The stator yoke 106 integral with the stator teeth 106a or 106b is fixed to each of two cases 103. Thus, two stators 102 corresponding to the two stator coils 105 for excitation are formed.
To one of the two cases 103 are fixed a flange 115 and a bearing 108, and to the other thereof is fixed another bearing 108. A rotor 109 consists of a rotor magnet 111 fixed to a rotor shaft 110, and a radial gap is formed between the rotor magnet 111 and the stator yoke 106 of each of the stators 102. The rotor shaft 110 is rotatably supported by the two bearings 108.
In the above-described conventional small step motor, the case 103, the bobbin 101, the stator coil 105, and the stator yoke 106 are arranged concentrically, which presents a problem in that the external dimensions of motor increase. Also, magnetic flux generated by the energization of the stator coil 105 mainly passes through an end face 106a1 of the stator tooth 106a and an end face 106b1 of the stator tooth 106b, which also presents a problem in that the magnetic flux is not applied effectively to the rotor magnet 111, so that the motor output does not increase.
To solve these problems, the applicant of the present invention has proposed a motor having a construction disclosed in Japanese Patent Application Laid-Open No. H9-331666. The motor relating to this proposal is constructed so that a cylindrical permanent magnet is equally divided in the circumferential direction to form a rotor (rotor magnet) in which the magnet is magnetized so as to have different poles alternately, a first coil, the rotor, and a second coil are arranged in that order in the axial direction of the rotor (the axial direction of the motor), a first outside magnetic pole portion and a first inside magnetic pole portion, which are excited by the first coil, are opposed to the outer peripheral surface and the inner peripheral surface on one side in the axial direction of the rotor, and a second outside magnetic pole portion and a second inside magnetic pole portion, which are excited by the second coil, are opposed to the outer peripheral surface and the inner peripheral surface on the other side in the axial direction of the rotor. In this construction, a rotating shaft, which is a rotor shaft, is taken-out of the cylindrical permanent magnet (magnet).
According to the motor having such a construction, the output can be increased, and also the external dimensions of motor can be made small. Further, by making the magnet thin, the distance between the first outside magnetic pole portion and the first inside magnetic pole portion and the distance between the second outside magnetic pole portion and the second inside magnetic pole portion can be shortened, by which the magnetic reluctance of a magnetic circuit can be decreased. Therefore, even if the current flowing in the first and second coils is small, a lot of magnetic flux can be generated, so that a high output can be maintained.
FIG. 8 is a schematic longitudinal sectional view of the motor having the above-described construction.
In FIG. 8, reference numeral 201 denotes the magnet, 202 denotes the first coil, and 203 denotes the second coil. A first stator 204 has the first outside magnetic pole portion 204a, 204b and the first inside magnetic pole portion 204c, 204d. A second stator 205 has the second outside magnetic pole portion 205a, 205b and the second inside magnetic pole portion 205c, 205d. An output shaft 206, to which the magnet 201 is fixed, rotates integrally with the magnet 201. The output shaft 206 is rotatably supported by bearing portions 204e and 205e of the first stator 204 and the second stator 205, respectively. Reference numeral 207 denotes a connecting ring for holding the first stator 204 and the second stator 205.
Also, the applicant of the present invention has proposed an improvement on the above-described motor in Japanese Patent Application Laid-Open No. H10-229670. In this improved motor, an inside magnetic pole is formed into a cylindrical shape, an output shaft inserted in an inside diameter portion of the inside magnetic pole is formed of a soft magnetic material, and a bearing, which is installed on a stator to rotatably hold the output shaft, is formed of a nonmagnetic material.
According to this proposal, the motor output is increased because the output shaft can also be used as a magnetic circuit. Also, the attraction between the stator and the output shaft caused by magnetism is prevented by the nonmagnetic material of the bearing.
However, the motors described in the aforementioned Japanese Patent Application Laid-Open Nos. H9-331666 and H10-229670 have a drawback in that the axial length of motor increases like the conventional step motor shown in FIG. 7A.
Also, for the motors shown in FIGS. 7A and 7B and FIG. 8, the position at which the magnetic flux generated by the energization of the first coil is applied to the magnet and the position at which the magnetic flux generated by the energization of the second coil is applied to the magnet shift from each other in the axial direction of the magnet. Therefore, when the magnet is unevenly magnetized at positions in the direction parallel with the shaft (the position on the side of 204 and the position on the side of 205 in FIG. 8), the accuracy of rotation stop position of the magnet sometimes decreases.
To solve this problem, the applicant of the present invention has proposed a motor described in Japanese Patent Application Laid-Open No. 2001-206302. This motor includes a rotatable rotor having a cylindrical magnet portion which is divided in the circumferential direction so as to be magnetized to different poles alternately; a first outside magnetic pole portion opposed to a first predetermined angle range of the outer peripheral surface of the magnet portion, which is excited by a first coil; a first inside magnetic pole portion opposed to the inner peripheral surface of the magnet portion, which is excited by the first coil; a second outside magnetic pole portion opposed to a second predetermined angle range of the outer peripheral surface of the magnet portion, which is excited by a second coil; and a second inside magnetic pole portion opposed to the inner peripheral surface of the magnet portion, which is excited by the second coil. The first outside magnetic pole portion and the second outside magnetic pole portion are arranged on the same circumference with the magnet portion being the center.
However, in the motor proposed in the aforementioned document, it is necessary to provide a predetermined gap between the inside diameter of magnet and the opposed inside magnetic pole, so that the control of the gap at the time of manufacture increases the manufacturing cost.
Also, regarding the shape of stator as well, the cylindrical inside magnetic pole portion and outside magnetic pole portion are needed, and it is difficult in terms of parts manufacture to integrally fabricate these magnetic pole portions. Further, in the case where after these magnetic pole portions are manufactured separately, they are assembled to each other, the number of parts increases, which leads to an increase in cost.